It is well known that the performance of cordless and cellular telephones, and other mobile transceivers is susceptible to hostile environments, such as multipath fading and interference from similar units, and noise from other sources. In order to establish a good communication link between the units of a transceiver, e.g., between a handset and its base unit, over a particular channel, it is important to know in advance whether that channel is "clean" or not, i.e., whether or not the channel is in use or subject to unacceptable levels of noise and/or interference. If a channel is found to be clean, then the frequency corresponding to that channel is used as the carrier of the information signals, which may be speech and/or data. If the channel is known to be in use or of insufficient quality, then the equipment looks for another channel to establish the communication link.
Many conventional cordless telephones with frequency-modulation (FM) schemes provide 25 or more different FM carriers. These cordless telephones typically include a circuit located in the handset that automatically seeks a clean channel and establishes an information link between the handset and base using the corresponding FM carrier. Upon establishing the information link, the base unit goes off-hook and talk-time begins. However, if during talk-time the selected channel should become subject to unacceptable levels of noise or interference, it is usually left to the user to manually seek a clean channel via a channel-switch button, which is normally located on the handset.
The procedure used by many conventional handsets to establish a communication link involves the steps of first measuring the strength of a received signal, including information, noise and interference, over one of the available channels, and then comparing that measurement to a predetermined threshold. If the threshold is exceeded, then that channel is ignored, since it is assumed that that channel is either busy (someone else is using that channel) or unacceptably noisy. The handset then moves to a different channel and the measurement and comparison steps are repeated. When a channel is found to have a measured signal strength that does not exceeded the predetermined threshold, the handset informs the base of the selected carrier frequency and, if the base approves, establishes an information link with the base which then goes off-hook. If the base does not approve of the chosen carrier frequency, the process is repeated.
Conventional cordless telephones usually include received-signal-strength-indicator (RSSI) circuits in both the handset and base. RSSI circuits typically conduct average energy measurements of an FM carrier, which includes the information portion (e.g., the speech and/or data), plus the interference and noise portions. The RSSI circuits also compare that average energy measurement, referred to as the RSSI level, to a predetermined RSSI threshold to determine whether or not the RSSI level exceeds the RSSI threshold. Many of the currently available cordless telephones routinely use an RSSI output in a channel searching procedure to check if a channel is free and clear before establishing an information link between the handset and the base. If the RSSI level is higher than the predetermined RSSI threshold, indicating that that channel is in use or otherwise not clean, then that channel is rejected and another channel is tested. This channel searching procedure recycles until a clean channel is found, at which time control circuitry in the handset will attempt to establish an information link with the base using the FM carrier corresponding to the clean channel. During talk-time, cordless telephones normally do not use an RSSI output, or any other indicator, to monitor the performance of the presently used channel to see if it is becoming noisy or unstable.
It is known that in a benign environment, the RSSI levels vary with the distance between the handset and the base, that is, as a user with a handset moves farther away from the base, the RSSI levels drop. Channel switching may even prove impossible, notwithstanding an RSSI output that appears to indicate that a particular channel is free and clean, i.e., the RSSI level for that channel is below the RSSI threshold.
As mentioned above, a user wishing to change channels during talk-time can usually do so by pressing a button on the handset. In response to such a user request, a control circuit will search for a new channel in accordance with the above-described procedure. Although circuits that perform a manual channel switch during talk-time have served the purpose, they have not proved effective under all conditions for a number of reasons. For example, in some cases, a manual channel switch during talk-time can prove impossible. Consider the case where a user with a handset is located far from the base (the RSSI level is relatively low) and a strong interferer appears on the user's channel. In that case, the user will not be able to manually switch to a new channel if the communication link between the user's handset and the base is so corrupted by the interferer that the base and handset cannot exchange the necessary control signals required to execute a channel switch. Moreover, a user with a corrupted communication link, as just described, may not even be able to transmit the appropriate control signals to the base that are required to hang the telephone up. Additionally, for many users, manual channel switching can be ineffective because these users either do not know how to perform a channel switch, or they find manual switching during talk-time to be cumbersome and annoying. Therefore, those concerned with the development of cordless and cellular telephones, and other mobile transceivers have recognized a need for a technique of automatically performing a channel search and switch in real time during talk-time. The present invention addresses this need.